In recent years, foam materials have been attracting attention in view of achieving lightweight. However, if a composition containing a resin or elastomer is merely foamed, the mechanical strength will decrease, causing problems such as deterioration and deformation after long-term use.
For this reason, such a composition is further crosslinked into a crosslinked foam, thereby finding a wide range of applications as a material that is lightweight and has a high mechanical strength, such as automobile parts, construction application materials, various packaging materials, and daily necessities.
Ethylene-vinyl acetate copolymers (EVA) are known as representative crosslinked foams; however, because EVA has a low melt tension, defoaming easily occurs, preventing the expansion ratio from increasing, resulting in an increased specific gravity. Furthermore, another problem thereof is that the defoaming causes significantly rough skin.
On the other hand, proposals have been also made regarding crosslinked foams using ethylene-α-olefin copolymers.
For example, Patent Document 1 discloses a crosslinked foam obtained by crosslinking and foaming an ethylene-α-olefin copolymer, using an organic peroxide, a crosslinking coagent, and a foaming agent.
Furthermore, Patent Document 2 discloses a crosslinked foam obtained by crosslinking and foaming a composition obtained by adding EVA and/or low density polyethylene to an ethylene-α-olefin copolymer.
Moreover, studies have been conducted on modifying the properties and improving the physical properties of a crosslinked foam by adding a styrene-based block copolymer to an ethylene-based copolymer. Patent Document 3 discloses a crosslinked foam obtained by adding a specific proportion of a styrene-based block copolymer to an ethylene-based copolymer.
However, in the conventionally proposed technique described above, it is difficult to obtain a crosslinked foam having a good balance of various physical properties. For example, in the case of the crosslinked foam disclosed in Patent Document 3, because the ethylene-α-olefin copolymer has a low hardness, when crosslinking is performed at a high expansion ratio, problems such as defoaming and deformation occur. Thus, a lightweight crosslinked foam is not obtained; or, even if a lightweight crosslinked foam is obtained, the mechanical properties and the like of the foam often significantly deteriorate. Accordingly, no methods have conventionally been proposed to prepare a crosslinked foam that is lightweight and has high mechanical properties at the same time.    Patent Document 1: Japanese Patent Laid-Open No. 2000-344924    Patent Document 2: Japanese Patent Laid-Open No. 2002-302565    Patent Document 3: Japanese Patent Laid-Open No. 2004-107519